If you ever try a demon control deck by urself you would take your word back. Rin, the First Disciple is not a Guarantee ticket, and it can be countered, silent & transformed. Furthermore, you have to play Azari, the Devourer for triggering its battlecry which mean you have to give up another powerful card SKULL OF THE MAN’ARI since two mechanics conflict each other.

Those cheap, dirty aggro deck is as good as dead. We don't want another brainless quest Rogue on the show.

Well if you're any good at control lock, you often need to ensure that you only play Rin when you can immediately destroy her, typically turn 7 with dark pact, and especially against control decks (particularly the polymorphing classes, mage and shaman).

First you have to pay 6 mana for a Fen Creeper (mediocre) with a special effect. And it has to die before being silenced, transformed or stolen.

Then you have to spend 5 mana for a 2/2 (laughable), then 5 mana for a 3/3 (awful), then 5 mana for a 4/4 (bad), then 5 mana for a 5/5 (meh), and then 5 mana for a 6/6 (alright).

Finally you have to play 10 mana for a 10/10 (slow as fuck) that empties their deck.

So it's not just "one card to pull off and you win." It's more than 4 turns worth of not doing anything other than playing stats, during which you have to stay alive and not get out-tempo'd.

It's probably the slowest win condition that ever existed, and it's not even guaranteed to win you the game (what good is their deck being empty if they can still kill you from board or hand).

You're right that it's extremely slow, such that it's unplayable in many matchups. However I think you're slightly misevaluating the value that Rin represents. You're right that a 2/2 for 5 is awful, ordinarily. However in the case of Rin, the 5 mana 2/2 is a card that came from nowhere. You didn't have to sacrifice a slot in your deck for it, and it's an extra 2/2 of stats value on top of the total face value of your deck. Same goes for the other 5 mana demon summons. Control matchups are all about outvaluing your opponent, and in the late game control decks (and even some midrange decks) will very frequently have turns where they only spend a couple of mana, or they play a removal card uselessly just so they don't burn a card next turn. In any case like this, in which you would have been wasting the mana not doing anything anyway, a 5 mana 2/2 is more like a 0 mana 2/2 (and 0 mana 3/3, 4/4, etc.). A card that generates so much consistent, raw value (even if that value is largely very understatted) can be very powerful in a slow enough matchup.

Just as I expected, this card is a ace in the hole in control matchups. The feeling I had the other day when a Raza Priest desperately played Velen for Tempo and Dragonfired an Acolyte to try to get his dk right before I ate his deck... Priceless stuff.